1. Field of the Invention
This invention relates to a method for forming a photomask, and more particularly to and a fabrication process for a multi-layer photomask, which has a pattern with a narrow line width and a small pitch between lines.
2. Description of the Related Art
In a photolithography process, photomask is the main tool used to transfer a circuit pattern onto a wafer, so the photomask consequently plays an essential role in semiconductor fabrications. The main part of the photomask is a planar, transparent substrate. A circuit pattern is formed by a chromium layer that is about several hundred .ANG. thick, which chromium layer includes a pattern desired to cover a portion of the transparent substrate. An anti-reflection layer is often formed on the chromium to prevent light from being reflected by the chromium layer. When the photomask is exposed to a light source, a circuit pattern is projected onto the wafer.
FIG. 1 is a cross-sectional view showing a conventional binary mask. A typical binary mask comprises a transparent substrate 10 and a defined blinding layer 12. A material of the blinding layer 12 comprises chromium, iron oxide (FeO.sub.x), silicon nitride or aluminum (Al).
Another conventional photomask is known as a "phase shifting mask (PSM)". Typically, there are two kinds of phase shifting mask. One is a strong PSM and the other is a weak PSM. The strong PSM further includes a Levenson and an alternating PSM. The weak PSM comprises a half-tone PSM, a rim PSM and an attenuated PSM. FIG. 2 is a cross-sectional view showing a conventional half-tone PSM (HTPSM). Normally, a HTPSM, which has a larger depth of focus than other types of PSM, is used to produce hole patterns for semiconductor devices on a wafer.
In FIG. 2, a defined shifting layer 22 is formed on a portion of a provided transparent substrate 20. Hole patterns are thus formed on the exposed transparent substrate 20. The shifting layer 22 can generate a 180.degree. phase shift in incoming light. The light transparency of this shifting layer 22 is roughly between 3-10%. The HTPSM operates by shifting incoming light through phase angles of 0.degree. and 180.degree., alternately.
The patterns of a photomask are drawn with an e-beam or laser. The size of pitches between the patterns is limited by the resolution of the e-beam or of the laser. One method of decreasing the pitches is to use an advanced machine to enhance the resolution but this method does increase the cost of forming a photomask.